Shear wave vibrational directions and related fault movements in southern California earthquakes:
Vibrational directions of direct shear waves from a number of small local earthquakes in southern California, recorded at Pasadena and Riverside, are determined and discussed in relation to corresponding faulting at the source. A theoretical relationship between small fault displacements and wave vibrational directions is proposed.Using this relationship, directions of SV and SH motions from various fault types at normal depths of focus are adduced. Observed initial shear wave motions indicate generally consistent SH, but usually less consistent SV displacements. Values SV/SH are consistent in a few localities, but generally vary widely. Polarization of SH waves is evidenced from the data; that of the SV is suggested, but is not so clearly indicated. Probably the entire shear wave is approximately plane polarized. From the data it appears that horizontal components of faulting in southern California, within local regions, usually take place in the same general direction. Vertical fault directions appear to vary. An analysis of faulting at the source, based on a comparison of observed SV and SH motions with theoretical shear motions and with observations by Gutenberg (1941) of impulses of compressional waves, is described. A fault pattern in agreement with the seismic data and also the regional surface geology involves primarily northwesterly trending right handed transcurrent faults in some parts of southern California, east west trending reverse or thrust faults in others, and coexistence of the two in a few localities. Somewhat simplified stress distributions in agreement with the data are discussed briefly. A method for determining strains of incident shear waves from linear strain seismograms, recorded through a new Benioff transducer and short period galvanometer, is also presented.
The relationship of the Modelo and Ridge Route formations in the Southern Ridge Basin, California:
The Modelo formation and the overlying Ridge Route formation, as exposed in the southern part of the Ridge Basin, are similar in general appearance and consist of alternating begs of sandstone and siltsone. The contact between the formations is neither a sharp break nor is it characterized by an interfingering of sedimentary units.The two units differ in general strike by as much as 35 degrees in the area studies, although the strike grades uniformly from that of one formation to that of the other.This difference in attitude is believed to be a depositional feature, rather than one resulting from structural disturbance.
The Modelo formation thins markedly in a northerly direction, and the Modelo sea appears to have transgressed the area from south to north during a part of upper Micoene time. Subsequently this sea retreated from the area, while non-marine Ridge Route beds appear to have been deposited concomitantly from a northerly direction. No appreciable time lapse is evidence between the periods of marine and non-marine deposition. The lowermost of the Ridge Route formation beds may be fluviatile deposits, but the remainder of the formation appears to be of lacustrine origin. In post-Pliocene time the region was folded into the Ridge Basin. The folding evidently was related to displacements along the adjacent San Gabriel fault.
